1. Field of the Invention
This invention relates to a Karman vortex flow meter used with internal combustion engines of vehicles, etc., for measuring the flow velocity and quantity of a fluid and in particular to a method using heating coils, etc., for detecting Karman vortexes.
2. Description of Related Art
The methods using heating coils for detecting Karman vortexes are those for detecting Karman vortexes occurring on a fluid directly by means of heating coils and for detecting a subflow produced by alternately occurring Karman vortexes.
The former method is described, for example, in Unexamined Japanese Patent Publication 57-86013 (1982) as shown in FIG. 7 and the latter is described, for example, in Unexamined Japanese Patent Publication 57-17864 (1982) as shown in FIG. 8.
In FIG. 7, a vortex generation element 102 is installed in a duct 101 where a fluid flows and heating coils 109a and 109b are placed symmetrically in a vortex pass area downstream from the vortex generation element 102. In the structure, Karman vortexes 107 produced by a fluid flow indicated by an arrow 110 are detected as an increase or decrease in a heating current fed into the heating coils 109a and 109b for measuring the flow velocity or quantity according to the Karman vortex period.
In FIG. 8, a heating coil is put in a through hole 250 piercing both sides of a vortex generation element 202 installed in a duct 101 where a fluid flows, and a flow produced in the through hole 250 as Karman vortexes 207 occur is converted into an AC signal synchronized with the vortex occurrence using a radiation amount change of the heating coil 209. The flow velocity or quantity is measured from the frequency or period of the AC signal.
The conventional Karman vortex flow meters introduce the following problems:
If heating coils are installed directly in the duct through which a fluid passes as shown in FIG. 7, when the fluid contains dust, the dust will accumulate in the heating coils as the flow meter is used for a long term. Since the accumulated dust hinders the heating coils in heat radiation, electric output of the heating coils decreases and in the end, Karman vortexes cannot be detected.
If two holes are made on the side faces of the vortex generation element and the through passage is disposed so as to connect the holes as shown in FIG. 8, Karman vortexes alternately occurring via the through passage interfere with each other, causing Karman vortexes to sometimes occur unstably.
To solve the problems, a Karman vortex flow meter as shown in FIGS. 9A and 9B has been proposed.
In FIGS. 9A and 9B, a vortex generation element 2 for generating Karman vortexes is installed in a duct 1 into which a fluid flows and detection passages 5 are disposed along the fluid flow in the duct 1. Each detection passage 5 has a flow outlet 4 formed in the end face of the vortex generation element 2 and a flow inlet 3 formed in the top wall face of the duct 1 upstream from the flow outlet 4. Further, heating coils 9 for detecting a flow velocity change in the detection passages 5 caused by a negative pressure of a Karman vortex and supports 8 for supporting the heating coils 9 are located in the detection passages 5.
The above mentioned Karman vortex flow meter is described in detail in a copending U.S. patent application No. 08/553,901, filed on Nov. 6, 1996.
The Karman vortex flow meter of the structure in FIG. 9 can prevent dust having a heavy specific gravity from flowing into the detection passages 5; however, if the fluid contains water drops or bubbles having a comparatively light specific gravity, the water drops or bubbles enter the detection passages. Residence of the water drops or bubbles can hinder the heating coils from detecting Karman vortexes or fuse the heating coils.